The combination of structural imaging with functional and molecular imaging allows functional or molecular information to be assigned to specific anatomical structures, such as tissues or organs, so that, ultimately, structure-function relationships can be recorded. Clinical examples already used include the combination of structural and functional magnetic resonance imaging, wherein areas of the brain for example which have been activated by specific excitation are superimposed with high-resolution anatomical images. Another example is the combination of positron emission tomography (PET) with computer tomography (CT), which makes it possible to identify areas of altered metabolic activity within the overall anatomical system. Image recording devices which combine magnetic resonance (MR) and PET are currently being tested in a clinical environment.
PET and SPECT (single photon emission computed tomography) are examples of nuclear medical imaging techniques which primarily map functional processes in an object under examination. With PET, images of living organisms are produced, which make it possible to visualize the distribution of a previously administered, weakly radioactively labeled substance in the organism, said substance having been enriched in the organism in such a way that biochemical and physiological processes can be mapped. Radionuclides which emit positrons during decay are used as the substance (tracer) in this case. After a short distance, for example 2 to 3 mm, the positrons interact with an electron, resulting in “annihilation”. Both particles (positron and electron) are destroyed, and two high-energy photons (gamma radiation) are formed with an energy of 511 keV each. These photons move away from one another at an angle of approx. 180°. Both photons are measured, for example using a detector ring, whereby two different points of the detector ring are contacted at the same time. It is possible to detect positron emission and to estimate the point of annihilation on the basis of the coincidence of the two measurements.
It has also been proposed to form systems which allow simultaneous nuclear medical imaging and fluorescence imaging (often also referred to as optical imaging). With fluorescence imaging, a fluorescent or bioluminescent substance is excited to fluorescence within the body using excitation light, whereupon light of a certain wavelength is emitted. This light can be detected, and therefore an image is ultimately formed which shows where the excited substance is located.
The combining of nuclear medical imaging with fluorescence imaging is motivated by the fact that different molecular targets are to be measured at the same time and that imaging techniques are sought which can be used as a kind of “translation platform” between the widely used optical imaging techniques, which use bioluminescent or fluorescent reporter substances or injected fluorescent substances, and nuclear medical examinations using a radiotracer. Tomographic imaging instruments for small animals, which combine optical imaging and PET, or optical imaging and SPECT with one another, have already been proposed in this regard. One possible clinical use of such hybrid technologies would be, for example, the use of the sensitive whole-body potential of PET or SPECT to carry out an “optical biopsy” using an endoscope or a catheter having an optical imaging device, so that, ultimately, it is possible to achieve local “mapping” of fluorescence signals with high resolution and high sensitivity at points which were already noticeable in the PET or SPECT scan.
Since magnetic resonance imaging offers good structural resolution, it has also been proposed to combine optical imaging, that is to say fluorescence imaging using bioluminescent or fluorescent substances, with magnetic resonance imaging. In this way, high-resolution three-dimensional structural imaging can be combined with optical imaging, wherein fields of application range from the imaging of small animals to the provision of recorded images of the human breast or the human brain. Since a spatially resolved three-dimensional reconstruction of the diffuse fluorescence images is ultimately impossible on this basis alone, the spatially recorded magnetic resonance images can be used to locate the boundaries of tissues having different optical properties and thus to increase the accuracy of three-dimensional fluorescence reconstruction.
Contrast agents which are suitable for imaging with a number of modalities are also being examined in conjunction with these hybrid modalities. A number of large biomolecules, such as peptides or proteins, and of particles, such as microbubbles, liposomes and nanoparticles, form suitable platforms for producing contrast agents which can provide a contrast for more than one imaging modality. The motivation for the development of these contrast agents lies in the fact that they allow examination of the same target using a single contrast agent on different imaging platforms and on different scales. For example, such a contrast agent suitable for a number of modalities can be administered to then carry out fluorescence imaging and to later carry out magnetic resonance imaging, PET or SPECT by way of the same contrast agent. Another possibility for the use of these contrast agents can be found in imaging devices which combine different imaging modalities. For example, in a combined PET/MR system, the high sensitivity of PET can be used to locate areas of high uptake of a PET/MR contrast agent in the body, whereupon high-resolution magnetic resonance imaging of said contrast agent is possible, wherein the magnetic resonance images must be recorded only in the areas where the PET signal was observed.
A large number of hybrid contrast agents which are suitable for optical imaging, that is to say fluorescence imaging, and for magnetic resonance have already been proposed. Examples include fluorescent quantum dots with a paramagnetic coating, quantum dots with high native relaxivity, lipoproteins containing iron oxide nanoparticles and quantum dots, liposomes containing gadolinium and fluorescence agents, as well as antibodies which are provided with both magnetic nanoparticles and fluorescence agents. In some cases these particles and proteins are additionally being designed to include radionuclides suitable for PET or SPECT imaging.
PET detector arrangements known in the prior art include, for example, an array of scintillator blocks, which convert the energy of incident gamma photons into low-energy photons of visible light. This visible light is then captured by a photodetector, which may be a CCD (charge coupled device) detector, an APD (avalanche photo diode) detector, or a CMOS (complementary metal oxide semiconductor) sensor, for example.
As already mentioned, image recording devices which are designed for simultaneous magnetic resonance and PET imaging are also already known in the prior art. A large number of designs are known. It has been proposed, for example, to divide the gradient coil of a magnetic resonance device into two halves, wherein the PET detector ring is provided in the gap between the two portions of the gradient coil arrangement. A radio frequency shield, which completely surrounds the radio frequency coil and blocks the radio frequency signals of the coil but does not substantially weaken the γ-photons of PET, separates the radio frequency coil (body coil) from the gradient coil arrangement and the PET detector ring.
A shell-like structure of a combined MR/PET device was proposed in another known device. In this case, the PET detector arrangement is provided as a tubular insert between the gradient coil arrangement and the radio frequency body coil. The radio frequency shield separates the body coil from the PET insert. The gradient coil arrangement defines the outer diameter of the PET insert. In the known MR/PET device, the PET detector arrangement consists of detector blocks with an LSO scintillator crystal and a highly sensitive APD photodetector array with associated electronics. The radio frequency shield and the conductors of the body coil consist of thin copper strips, which are almost completely transparent to the 511 keV photons of PET.
Another combined image recording device, in which a PET detector arrangement is integrated into a magnetic resonance device, is known from U.S. Pat. No. 7,719,277. This concerns a compact solution, in which the PET detector blocks of the PET detector arrangement have gaps in which the longitudinal conductors (often also called “rods”) of the coaxially disposed radio frequency coil arrangement are guided. The detector unit, which consequently contains the body coil and the PET detector arrangement, is formed as a tubular insert which is separated from the gradient coil arrangement by the radio frequency shield. The radio frequency shield is folded over laterally to shield the PET electronics from the radio frequency coils of the body coil.
An image recording device having two imaging modalities is also known from WO 2008/028904 A1. It is proposed to provide a magnetic resonance device having at least one optical imaging detector, wherein the magnetic resonance data and the optical image data of an object are to be recorded at the same time. The magnetic resonance device comprises a magnet for generating a static magnetic field in an imaging volume of the magnetic resonance device, gradient coils for generating magnetic gradient fields, and a radio frequency coil which is arranged within the imaging volume so as to surround an object. At least one subsystem of the at least one optical imaging detector is to be arranged within the imaging volume, so that optical photons emitted from the object to be imaged can be received through an opening in the radio frequency coil. The microlens array used for this purpose is to be integrated into the radio frequency coil.
Starting from this prior art, the object of the present invention is to create an image recording device which allows progressive advantageous combination of different imaging modalities.